Conventionally, some diesel engines have selective reduction catalyst incorporated in an exhaust pipe through which exhaust gas flow, said catalyst having a feature of selectively reacting NOx with a reducing agent even in the presence of oxygen; a required amount of reducing agent is added upstream of the catalyst to be reacted with nitrogen oxides (NOx) in exhaust gas on the catalyst to thereby reduce a concentration of the discharged NOx.
Meanwhile, effectiveness of ammonia (NH3) used as a reducing agent for reduction and purification of NOx is well known in a field of industrial flue gas denitration, for example, in a plant. However, in a field of automobile where safety is hard to assure as to running with ammonia itself being loaded, researches have been made nowadays on use of nontoxic urea water as the reducing agent (see, for example, Reference 1).                [Reference 1] JP 2002-161732A        
More specifically, addition of the urea water to the exhaust gas upstream of selective reduction catalyst under a temperature condition of about 170-180° C. or more causes the urea water to be decomposed into ammonia and carbon monoxide, and NOx in the exhaust gas on the catalyst is satisfactorily reduced and purified by ammonia.